In the art of vibrating element angular rate sensors it is well known that a vibrating mass subjected to an angular rate of turn experiences gyroscopic forces whose effects, such as deflections, forces or stresses, can be utilized as an indication of the magnitude of a turn rate.
Many embodiments utilizing this effect have been employed to various degrees of success. Examples of some of the different shapes employed include the traditional tuning fork shape of U.S. Pat. No. Re 22,409 to Lyman, the cantilever beam type of U.S. Pat. No. 4,267,731 to Jacobson, the vibrating wire shape of U.S. Pat. No. 3,903,747 to Johnson, the coaxial tuning fork of U.S. Pat. No. 4,802,364 to Cage, the free beam type of U.S. Pat. No. 4,836,023 to Oikawa, and the hollow cylinder type of U.S. Pat. No. 4,759,220 to Burdess. Each of these different shapes utilizes the same well known concept of a lightly damped mass/spring system designed to vibrate in a prescribed mode shape.
As is well-known in the art, when such a system is vibrated in a prescribed mode shape and is subjected to an angular rate of turn about certain axes, it experiences gyroscopic forces which can cause measurable system effects indicative of the rate of turn. These resonant systems are designed to enhance these measurable effects which are usually exemplified as forces causing deflections and/or stresses and which can be directly measured as an indication of the angular rate of turn.
A vibrating angular rate sensor provides superior performance and longer service life for less cost over the traditional spinning rotor-type rate sensors due to its inherent simplicity, reduction of the total number of parts, elimination of wearing and temperature sensitive parts, etc. Yet, to date, the standard spinning rotor type angular rate sensor is still the most used technology for this purpose.
One of the major problems associated with attaining accurate measurements from a vibrating element angular rate sensor has been the isolation of the vibrating element from influences of its environment such as ambient vibration and changes in the device mounting or boundary conditions. This problem is due, in part, to the fact that this type of apparatus is designed to be a highly resonant structure with a minimum of damping and can, therefore, vibrate in not only the desired "driven" mode of vibration but also a number of other modes of vibration. The influence of ambient vibration or changing mounting conditions can excite these other modes of vibration to cause disturbances on to the driven mode. If these disturbances are synchronous with the driven mode by being at or near its frequency or one of its harmonics, errors can occur in the resultant turn rate signals. If these disturbances are non-synchronous with the driven mode, then the result is usually noise in the resultant rate signals. Therefore, all natural modes of vibration that occur on the structure except for the driven mode represent potential sources of error or noise in the resultant rate signals.
Traditionally, the vibrating elements used on this type of device have been very simple shapes such as straight wires, cantilever beams, fixed beams, and others. Each of these shapes can vibrate in any of a number of well-defined natural modes of vibration. It is generally not possible to eliminate any one natural mode of vibration from such a simple structure. It is usually only possible to change some of its characteristics such as frequency, mode shape or damping. The best design strategy for this type of device is to place the natural frequencies of all other modes of vibration safely away from that of the driven mode and its harmonics.
It is, therefore, recognized that the ability to control the frequency of the naturally occurring modes of vibration and the separation of those modes from harmonics of the driven frequency would represent a great improvement in the art.
Single element vibrating angular rate sensors are particularly sensitive to ambient vibration and changing mounting conditions because reaction forces from the vibration of the element are applied directly to its attachment to the sensor's mounting structure. This allows vibrational energy to escape to the environment and ambient vibrational energy to enter and disturb the vibrating element. Multiple element vibrating angular rate sensors, such as tuning forks, were conceived as an attempt to circumvent the single element problem by vibrating one element in opposition to another element to isolate the vibrational energy and keep the reaction forces balanced and inside the vibrating system. This reduces the influence of the mounting conditions and ambient vibration from having an effect on the vibrating apparatus. Although the concept of using multiple vibrating elements has been implemented by many embodiments with good results, another problem was created which was the more than doubling of the natural modes of vibration into "in-phase" and "out-of-phase" modes and the necessity to control these additional modes.
For clarity, the term "out-of-phase mode" is defined as a deflected shape or mode shape in which the reaction forces from the deflection of a first element are opposed by substantially equal and opposite forces from another element of the apparatus with the result that substantially no net reaction forces are applied to the common mounting or boundary conditions of the apparatus. The term "in-phase mode" is defined as a deflected shape or mode shape in which the reaction forces from the deflection of a first element are added to substantially equal forces from another element of the apparatus with the result that the total reaction force is applied directly to the common mounting or boundary conditions of the apparatus.
U.S. Pat. No. 4,802,364 to Cage et al. entitled "Angular Rate Sensor" discloses an angular rate sensor where tynes vibrate in a coaxial manner. Both the driven mode of vibration and the reaction mode of vibration are balanced with the mass spring elements vibrating in opposition to each other for both directions.
Although the balancing of the vibration for both the driven and the reaction directions represents an important forward step, the problem of mode separation is left unaddressed. Further, in order for the tynes of this arrangement to be well coupled to each other, the common base needs to be flexible in relation to the tynes. In other words, the more rigid the common base, the less each tyne "feels" the effect of its opposing tyne and they can therefore lose track of each other's timing or phase relationship. In the extreme case the tynes could vibrate independently since they would have no effect on each other. On the other hand, if an excessively flexible common base is used, the necessary mounting pedestal which attaches the device to the rest of the world would interfere with this flexibility and therefore creates an undesirable design trade off between coupling and mounting parameters.
It is one object of the present invention, therefore, to provide a unique structure and measurement technique which is highly immune to the affects of ambient vibration and changing mounting conditions while maintaining a high degree of flexible coupling between the vibrating elements and at the same time allowing for a prescribed amount of separation between the driven mode of vibration and its harmonics from other undesired modes of vibration.
Another problem associated with vibrating element angular rate sensors is that of creating motion drivers and detectors that accurately drive and sense the vibrational movement without influencing or disturbing the resultant signals. Methods previously employed for this purpose include piezoelectric bending element sensors and drivers, capacitive plate sensors and inductively coupled drivers and sensors. Each of these methods can have adverse affects on the vibrational characteristics of the instrument by creating forces between the elements of the motion sensors or drivers through inductive or capacitive coupling. Also, the connection of wires or electrodes to the vibrating elements can add structural characteristics which can adversely affect the vibrating characteristics of the instrument and can cause errors in the resulting signals. These wires and electrodes are subjected to the vibration of the tynes and can also fatigue, causing total failure of the apparatus.
A second object of this invention is to provide a means whereby an angular rate sensor can be created with motion drivers and motion detectors that accurately drive and sense the vibrational movement of the elements without introducing structural characteristics or forces that adversely affect the vibration of the structure or the resultant signals therefrom. In addition, since these motion drivers and detectors are not subjected to the vibration of the elements, the arrangement eliminates the possibility of fatigue failure of these components.
A third problem associated with vibrating angular rate sensors is that of reducing the influence of unwanted resonances on the structure. Traditionally vibrating element angular rate sensors are designed to operate as lightly damped or highly resonant systems. Certain types of input motion can start undesired modes freely vibrating adding unwanted beat or noise frequencies to the resultant signals that fail to die out in an acceptable length of time.
It is a third object of this invention, therefore, to provide a means for creating a vibrating angular rate sensor which can quickly reduce unwanted vibrations without introducing errors on the resultant signals.
It is another object of the invention to provide a means for controlling changes in the distance between two objects, at least one of which is moveable, by controlling the strength of a magnetic field existing between the two objects.